Method and apparatus for displaying the internal structure of an object

ABSTRACT

The pattern of X-rays produced by interaction of a beam of X-rays and a  ted object is converted to an optical signal by a fine-grained zinc silicate screen which is viewed by a television camera through a magnifying optical system. The combination of a fine-grained luminescent screen of high resolving power with a camera having a photosensitive surface of much lower resolving power and high sensitivity provides real-time images of structural details not heretofore visualized except by photographic processes.

This invention relates to direct display topography and fluoroscopy, and particularly to a method of displaying a visible image of the internal structure of an object by means of a beam of radiation, and to apparatus capable of performing the method.

In its more specific aspects, the invention relates to topography and fluoroscopy by means of a beam of X-rays and like radiation which passes through the tested object or is reflected or diffracted by the object.

Known systems of X-ray topography provide images of the fine structure of the tested object by two basic processes. In one known process, the radiation modified by interaction with the object is intercepted by a photographic emulsion which produces the desired visual image after processing. In the other process, the pattern of secondary radiation transmitted or emitted from the object is received by the photosensitive surface of a television camera and displayed on a monitor screen. The photographic process is capable of a resolution of 1 μm or 1000 lines per millimeter, but requires exposure times of approximately one hour to about 30 hours (see "Advances in X-Ray Analysis", vol. 10, 1967, pp. 1-8). Electronic display of the secondary beam pattern is available from a vidicon tube in which a charge density pattern is formed by photoconduction and stored on a photoconductor surface that is scanned by an electron beam (Applied Physics Letters, vol. 13, No. 11, Dec. 1, 1968, pages 387-389). While the electronic system instantaneously displays changes occurring in the tested object, the photosensitive surface cannot resolve details smaller than about 30 μm, such as are essential in crystal growth, formation and movement of dislocations. More recently, an electronic process achieving resolutions of slightly less than 25 μm has been disclosed, but still cannot approach the resolving power of the best photographic processes (Japanese J. of Applied Physics, vol. 11, No. 10, Oct. 1972, pages 1514-1521).

It is the primary object of this invention to provide a real-time display of the internal structure of a tested object at a resolution which comes closer to that available from the best photographic processes than the known electronic display systems.

According to a basic feature of this invention, the secondary beam of radiation generated by an object exposed to a suitable primary beam is intercepted by a luminescent screen of high resolving power which responds to the secondary beam by emitting a finely detailed visible image. This image is optically magnified and the magnified image is received by the photosensitive surface of a television camera to produce an electronically enlarged monitor image of the scanned photosensitive surface. Because of the intervening optical magnification, the relatively low resolving power of the photosensitive surface in the television camera is adequate to reproduce all detail capable of being resolved by the luminescent screen.

The invention is applicable to the non-destructive examination of objects by transmitted or reflected X-rays, but is suitable, in its basic aspects, to the investigation of materials by other high-energy radiation including ultraviolet light, gamma rays, and electron beams. It permits the instantaneous observation of changes in structures smaller than 10 μm.

The spacing between the tested object and the luminescent screen should be as small as possible for best resolving power and high sensitivity, and should preferably be no greater than is necessary to permit separation of the secondary beam of modified radiation from the portion of the primary beam which continues at an angle from the secondary beam after interaction with the tested object. The primary beam includes wavelengths or lines characteristic of the target material in the X-ray tube employed, principally of wavelengths K.sub.α.sbsb.1 and K.sub.α.sbsb.2, which produce divergent respective secondary rays. The screen must be close enough to the tested object that the divergence of these principal components of the secondary beam is too small to be resolved by the luminescent screen. The sensitivity of the process according to the invention is thus enhanced by the use of both components of the K.sub.α doublet without loss of resolution. It is further enhanced by the close spacing between the source of radiation and the examined object, and by the close spacing between the object and the luminescent screen.

The invention permits the direct observation of changes occurring in crystalline solids such as phase transformations, diffusion, thermal changes, and changes due to the implantation of ions, to elevated pressure, to magnetic fields, light, and other external factors. The formation of lattice defects and the growth of crystals are readily visualized. Processes in the manufacture of integrated circuits and other semi-conductor devices are capable of visual representation. Other applications will readily suggest themselves.

An essential element of this invention is the luminescent screen which produces a visible image in response to the secondary radiation emitted from the tested object. The known luminescent screens consist of a phosphor layer on a carrier sheet. The most widely employed phosphors consist essentially of zinc sulfide and a small amount of an activator which determines the color of the visible light generated in response to incident ionizing radiation. Glass and cardboard are the usual carrier materials, and their phosphor coated faces are directed toward the source of primary radiation. The known luminescent screens are not suitable for the purpose of this invention because their resolving power has a practical limit at about 25 μm. This is insufficient for visualizing small cracks or non-metallic inclusions in metallic structures and even less adequate for visualizing fine structure.

It has been found that the luminescence of zinc sulfide phosphors is reduced sharply when they are comminuted to a grain size smaller than about 25 μm, whereas zinc silicate (Zn₂ SiO₄) can be ground to a particle size of much less than 5 μm without significant loss of luminosity, and that small amounts of manganese and rare earth metal compounds enhance the luminescent response of fine grains of zinc silicate phosphor to incident ionizing radiation. Resolutions of more than 500 lines per mm (less than 2 μm) are readily achieved.

The carriers employed for the purpose of the invention should be practically transparent to X-rays so that the phosphor-coated carrier surface may be directed toward the optical magnifying system and the photosensitive surface of the associated television camera. Suitable carriers include beryllium and plastic films or foils. Even these materials should not be used in sheets thicker than 100 μm, and a thickness of 6 μm or less is preferred for plastic foils. Among commercially available plastic foils, polyethylene terephthalate is preferred because of its great mechanical strength.

A luminescent screen having the desired resolving power is obtained by depositing a layer of finely ground zinc silicate phosphor on one of the two major faces of the carrier. The layer may be formed by sedimentation from a suspension of the phosphor particles in alcohol, by vapor coating, by cathode sputtering, or from an aerosol.

Other features and many of the attendant advantages of this invention will readily be appreciated from the following detailed description of preferred embodiments when considered in connection with the appended drawing in which:

FIG. 1 illustrates apparatus for X-ray topography according to the invention in conventional symbols;

FIG. 2 shows fluoroscopic apparatus of the invention in a view corresponding to that of FIG. 1;

FIG. 3 illustrates another apparatus of X-ray topography according to the invention; and

FIG. 4 shows a fluorescent screen of the invention suitable for use in each of the devices of FIGS. 1 to 3.

Referring initially to FIG. 1, there is shown an X-ray tube 1 producing a narrow diverging beam 2 of X-rays. An auxiliary, concavely bent quartz crystal 3 having a radius of curvature of 750 mm, diffracts or reflects the beam 2 whose angle of incidence is approximately 20°, as measured at the central rays of the beam 2, as a bundle 4 of parallel rays which is directed at a small acute angle to the surface of a specimen to be investigated, the illustrated specimen being a plate-shaped silicon crystal.

Because of the parallel alignment of the X-rays by the curved crystal 3, the specimen 5 may be arranged very close to the source of X-rays and the loss of radiation intensity, proportional to the square of the distance, is held to a minimum. The quartz crystal is so oriented that the second order Bragg reflection is preferentially employed. A curved mica crystal could be utilized for the same purpose and oriented for preferred Bragg reflexion in the fifth order mode. A known device for converting a diverging beam of X-rays into a bundle of parallel rays relies on double asymmetrical reflexion on silicon crystals (J. Phys. Soc. Japan 30, No. 4, April 1971, pp. 1136-1144) and may be employed with this invention instead of the curved quartz or mica crystals.

The X-rays reflected from the specimen 5 are intercepted by a fluorescent screen 6 whose luminous image is projected by an enlarging lens system 7 on the photosensitive surface of a television camera 9.

The screen 6, shown in more detail in FIG. 4, has a base 32 of polyethylene terephthalate foil, 3 μm thick, carrying a layer 33 of zinc silicate phosphor. In preparing the screen, zinc silicate activated with manganese was ground in a ball mill to a particle size of 5 μm and less. The particles were suspended in ethanol and deposited on the polyester foil 32 in a centrifuge. The screen 6 was mounted in the topographic apparatus in such a manner that the base 32 faced the X-rays 4 reflected from the specimen 5.

The polyester foil did not significantly weaken the incident X-ray radiation, and did not interfere with the inherent resolving power of the fluorescent layer of 200 lines per mm or better. It could be replaced without loss of these advantages by a beryllium foil not thicker than 0.1 mm. Zinc silicate has been found uniquely capable of being comminuted to the small grain size which provides the resolving power necessary for the purpose of this invention without losing its sensitivity to incident radiation, and without loss of yield.

The photosensitive surface 8 of the television camera 9 has a resolving power of only about 40 lines per mm (25 μm), but the integral sensitivity of the camera is high, approximately 150 micro amperes per lumen. The magnification power of the lens system is approximately 6× so as to match the resolving power of the screen 6 to that of the surface 8. Depending on the properties of the conventional television camera employed, optical magnifying systems having a magnifying power between 3× and 25× have been used to advantage.

The output of the camera 9 is monitored on the screen 11 of a television receiver 10.

The fluoroscopic apparatus shown in FIG. 2 has many basic elements in common with the apparatus described with reference to FIG. 1. The beam 2 of X-rays emitted by the tube 1 directly strikes a specimen 5 and the secondary beam 4' transmitted by the specimen and modified by its radiopaque features is intercepted by a fluorescent screen 6 which may be identical with the screen 6 described above. The television camera 9, identical with that described with reference to FIG. 1, has a sensitivity equal to or better than 150 microamperes per lumen. A magnifying lens system 7 is interposed between the screen 6 and the photosensitive surface 8 of the television camera. The further magnified image is displayed on the screen 11 of a monitor 10.

Penetrating radiation other than X-rays may be employed in the apparatus of FIG. 2 for investigating the internal structure of suitable specimens in an obvious manner, and a visible image is produced by the screen 6 when the screen intersects a beam of ultraviolet light, γ-rays, or electrons which has been modified by passage through a specimen in a manner obvious from FIG. 2.

The sensitivity of the screen employing a zinc silicate phosphor can be enhanced by cooling, and the optimum operating temperature is readily ascertained for any given set of conditions. Under most circumstances, however, zinc silicate screens perform well in this invention over the entire range of ordinary room temperatures. Limited tests indicate that screens carrying a single crystal of zinc silicate have adequate resolving power. Such single crystals of suitable shape are prepared by the method applied by Takei et al. (J. Cryst. Growth 23 121 [1974]) to the preparation of magnesium silicate crystals.

In the apparatus of the invention shown in FIG. 3, the beam of primary X-rays emitted by an X-ray tube 21 is limited by two rectangularly intersecting slits in respective layers of a stop 22 to a cross section of approximately 0.3 mm × 1.5 mm. The restricted beam impinges on the specimen 25 to be inspected, in this instance a plate shaped silicon crystal whose lattice planes are indicated by the lines of the hatching.

The crystal 25 is mounted on a carrier 25a which may be shifted at specified angles to the X-ray beam as indicated by the double arrow A. A lead shield 28, opaque to X-rays, is arranged on the far side of the crystal 25 to absorb the primary X-rays transmitted in line by the crystal 25, and not diffracted on the lattice planes. Only the secondary beam of diffracted rays is intercepted by a fluorescent screen 26. The luminous image produced by the screen 26 is enlarged by a lens system 27 and projected on the photosensitive surface of a television camera 29. The further enlarged image can be viewed on the screen 24 of a television receiver 23.

For best results, the specimen to be investigated should be as close as possible to the source of X-rays 1, 21 and to the luminescent screen 6, 26. The close coupling of the specimen and the source of radiation improves the effective intensity of the X-rays. The actual distance is determined largely by structural features of the X-ray tube employed. It may be as small as 100 mm with some commercially available tubes (rotary anode tube RU 500 of Rigaku Denki, Tokyo, Japan). Best results are obtained when the primary beam of X-rays is emitted from the target face in the tube at an angle of 5° - 10°, preferably 8°. The maximum distance between the specimen and the screen 6, 26 is essentially determined by the resolving power of the luminescent screen. The principal characteristic lines of the diffracted radiation, that is, the K.sub.α.sbsb.1 and K.sub.α.sbsb.2 lines, diverge from the specimen and must be intercepted before they can produce separage images when their divergence exceeds the resolving power of the screen.

The distance of the lead shield 28 from the specimen in the direction of the primary beam should be as small as possible, but must be chosen completely to separate the residual primary beam from the secondary radiation. The point at which the two beams are separated must be spaced from the specimen at least a distance of x mm which may be calculated from the equation ##EQU1## wherein W is the transverse width of the primary beam in millimeters, θ is the Bragg angle defined by the primary beam and the diffracting lattice planes of the tested crystal, and d is the thickness of the crystal in the direction of the primary beam in millimeters.

For best results, the distance of the lead shield 28 from the crystal 25 should not exceed 5 mm. For an X-ray tube having a molybdenum target, for a primary beam having a width of 0.4 mm, and for a silicon crystal 0.5 mm thick, the minimum distance x calculated from the above formula for refraction from the 220 plane is 2.63 mm, and the two components of the K.sub.α doublet diverge by less than 3 μm at this distance. The screen 26 should be as close to the shield 28 as possible, distances of less than 0.2 mm, preferably 0.05 - 0.1 mm, being both feasible and advantageous.

The stop 25 is preferably placed contiguously adjacent the window of the X-ray tube, and the specimen holder 25a is located no farther from the stop than is necessary to provide space for proper orientation of the selected lattice planes of the crystal 25 relative to the K.sub.α radiation. The distance between the tested crystal 25 and the screen 26 should not be greater than 3 mm if the resolving power of the screen is better than 5 μm (200 lines per mm).

The linear magnification of the screen image achieved by the lens system 27 is preferably four or greater. The television camera is a vidicon employing a conventional EIC (electron induced conductivity) tube.

The apparatus shown in FIG. 3 and described above permits the display of a crystal section 0.5 mm × 2 mm at 60× total magnification within 1/25 second, that is the time required by the camera for scanning an image frame. In a crystal having almost 1000 dislocations per cm², all dislocations were visible. There were no double images.

As compared to known systems of X-ray topography, the apparatus shown in FIG. 3 enhances the intensity of the diffracted beam by the close spacing of the structural elements, and by the utilization of both the K.sub.α.sbsb.1 and K.sub.α.sbsb.2 lines. The image amplifying power of the television camera is fully utilized without impairing the reproduction of fine structure that is not visualized when the photosensitive surface of the camera directly receives the X-rays.

It should be understood, of course, that the foregoing disclosure relates only to preferred embodiments of the invention, and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purpose of the disclosure which do not constitute departures from the spirit and scope of the invention set forth in the appended claims. 

What is claimed is:
 1. A method of displaying an image of the internal structure of an object which comprises:a. exposing said object to a primary beam of radiation sufficient to generate emission of a secondary beam of radiation from said object; b. intercepting said secondary beam by a luminescent screen responsive to said secondary beam to emit a visible image of said object; c. optically magnifying said image; d. receiving the magnified image on the photosensitive surface of a television camera; e. scanning said surface; and f. producing an electronically enlarged visual image of the image on said scanned surface,1. the resolving power of said screen being greater than the resolving power of said surface.
 2. A method as set forth in claim 1, wherein said primary beam of radiation is a beam of X-rays, and said object is a crystal having a lattice plane and capable of diffracting a portion of said primary beam, said secondary beam consisting of the diffracted portion of said primary beam, and wherein the remainder of said primary beam passes in a straight line through said crystal and is separated from said secondary beam at a point spaced from said crystal, the spacing of said point from said crystal being at least x mm, but not more than 5 mm, x being defined by the equation ##EQU2## wherein W is the transverse width of said primary beam in millimeters, θ is the Bragg angle defined by said primary beam and said plane, and d is the thickness of said crystal in the direction of said primary beam in millimeters.
 3. A method as set forth in claim 2, wherein said primary beam has characteristic components of wavelengths K.sub.α.sbsb.1 and K.sub.α.sbsb.2 respectively, said components producing divergent respective secondary rays, said spacing of said screen from said crystal being small enough to make the divergence of said secondary rays too small to be resolved by said screen.
 4. A method as set forth in claim 3, wherein said primary beam is being separated from said seconary beam by being intercepted by a shield opaque to said X-rays.
 5. A method as set forth in claim 3, wherein said primary beam is emitted from the metal target of an X-ray tube at an angle of 5° to 10° to the emitting face of said target.
 6. A method as set forth in claim 3, wherein the power of resolution of said screen is at least 200 lines per millimeter.
 7. A method as set forth in claim 6, wherein the spacing of said screen from said crystal is smaller than 3 millimeters.
 8. A method as set forth in claim 7, wherein said screen consists essentially of a carrier and of a layer of zinc silicate on said carrier as a phosphor.
 9. A method as set forth in claim 8, wherein said zinc silicate contains an amount of manganese sufficient to activate said phosphor.
 10. A method as set forth in claim 7, wherein said television camera has a sensitivity of at least 150 microamperes per lumen.
 11. A method as set forth in claim 3, wherein said primary beam is generated by refracting incident X-rays from an auxiliary crystal having an arcuate refracting surface.
 12. A method as set forth in claim 11, wherein said auxiliary crystal consists of mica or quartz.
 13. A method as set forth in claim 12, wherein the Bragg angle between said surface of said auxiliary cyrstal and said incident X-rays is 20°.
 14. A method as set forth in claim 1, wherein the radiation of said primary beam is γ-radiation, ultraviolet radiation, or electron radiation.
 15. Apparatus for displaying an image of the internal structure of an object which comprises:a. a source of a primary beam of radiation; b. object supporting means for supporting said object in a position in which the object is exposed to said primary beam for generating a secondary beam of modified radiation in response to said primary beam; c. a luminescent screen positioned for intersecting said modified radiation and capable of producing a visible image in response to the intersected radiation; d. a television camera having a photosensitive surface; e. optical means interposed between said screen and said surface for magnifying said visible image and for projecting the magnified image on said surface; and f. image producing means connected to said camera for electronically reproducing said magnified image, the resolving power of said screen being greater than the resolving power of said surface.
 16. Apparatus as set forth in claim 15, wherein the linear magnification of said visible image by said optical means is approximately equal to the ratio of the resolving powers of said screen and of said surface respectively.
 17. Apparatus as set forth in claim 15, further comprising shielding means interposed between said supporting means and said screen for shielding said screen form said primary beam without interfering with said secondary beam.
 18. Apparatus as set forth in claim 15, wherein said source includes an X-ray generating tube and collimating means interposed between said tube and said object supported on said supporting means.
 19. Apparatus as set forth in claim 18, wherein said screen is spaced not more than 3 millimeters from said object in said position.
 20. Apparatus as set forth in claim 15, wherein the resolving power of said screen is at least 100 lines per millimeter.
 21. Apparatus as set forth in claim 20, wherein said resolving power of the screen is at least 500 lines per millimeter.
 22. Apparatus as set forth in claim 20, wherein said screen includes a carrier and a layer of zinc phosphor on said carrier.
 23. Apparatus as set forth in claim 22, wherein said phosphor consists essentially of zinc silicate of the formula Zn₂ SiO₄ and trace amounts of manganese.
 24. Apparatus as set forth in claim 22, wherein said phosphor consists essentially of particles not greater than 5 μm.
 25. Apparatus as set forth in claim 20, wherein said television camera has a sensitivity of at least 150 microamperes per lumen.
 26. Apparatus as set forth in claim 15, wherein said screen consists essentially of a carrier sheet having two major faces and a thickness not greater than 100 μm, said carrier sheet being substantially transparent to said modified radiation, and a layer of particulate phosphor on one of said major faces, the other major face being directed toward said secondary beam.
 27. Apparatus as set forth in claim 26, wherein said carrier sheet consists essentially of a film of polyethylene terephthalate having a thickness not greater than 6 μm.
 28. Apparatus as set forth in claim 26, wherein said phosphor consists essentially of particles of zinc silicate not greater than 5 μm. 